This invention relates generally to air data properties of air vehicles, and more specifically, to methods and systems for providing air data parameters using mass flow and pressure sensors.
Air data properties are important in maintaining optimum performance of air vehicles. Three air data parameters include altitude, velocity, and mach number. Pressure altitude is a function of static pressure and is used to determine a quantity of flight conditions. For example, a decrease in pressure is typically indicative of an increase in altitude. Also, as pressure decreases, air density decreases. Air density is also a function of static air temperature. For example, if one plane is flying at sea level and one is flying at 10000 feet they both could be indicating 300 knots but the higher vehicle is actually flying faster. This phenomenon becomes more pronounced at higher altitudes.
Velocity is a function of impact pressure (i.e. the total pressure minus the static pressure). Velocity is the most common parameter used to control air traffic and other aircraft maneuvers. Velocity, as well as air density, is used to control fuel consumption and required power needed to fly at cruise conditions.
The mach number is the ratio of air vehicle speed and the speed of sound. As the air vehicle moves through the air, the air molecules near the air vehicle are disturbed and move around the air vehicle. If the air vehicle is moving at a relatively low speed, for example, less than 250 mph, the density of the air flow remains relatively constant.
At higher air vehicle speeds, some of the energy from the air vehicle compresses the air and locally changes a density of the air. This compressibility effect alters the amount of the resulting force on the air and becomes more important as speed increases. Near and beyond the speed of sound, about 330 meters per second or 760 mph at sea level, small disturbances in the flow are transmitted to other locations. Such disturbances have a constant entropy. For example, a sharp disturbance may generate a shock wave that could affect both the lift and drag of the air vehicle. As a result, the mach number is an important air data parameter that is used to control the performance of the air vehicle. The mach number also changes as a function of altitude.
As such, the air data parameters are very important in large aircraft and it may be considered flight critical. These parameters provide precision inputs for flight control. The air data computers that monitor these parameters tend to be high cost items because of accuracy requirements and complexity. For example, some air data systems are configured to be dual and triple redundant and incorporate very high precision and accurate pressure sensors. Extensive characterization procedures are also required to assure the air data computer system meets the desired performance levels.
In smaller air vehicles such as targets, missiles, and unmanned air vehicles (UAVs), the accuracy of the air data parameters is typically less critical. However, they are still important if air vehicle optimum performance is to be achieved. These air vehicle types are typically relatively low in cost and as such, any air data system incorporated therein must also be low cost.